Uncovering the roles of laser action modes in surface mechanical properties of 2024 aluminum alloy

The surface state/performance after laser action is as essential as the interplay mechanism between laser and material. Through the designed experiments, the effect of distinct laser action modes on the evolution mechanism of aluminum alloy surface's microstructure and mechanical property was discussed. Under laser-induced thermal effect, the procedure of "vaporization - plasma plume - recoil effect - solidification" occurred. The grains grew longitudinally with an average size of 13.49 mu m, which is smaller than that of substrate. Under the thermal oxidation, new oxides were formed. However, laser-induced force effect did not purpose a change in the composition. Importantly, force effect refined the grains to an average grain size as low as 6.21 mu m. Notably, an anomaly was discovered. The thermal effect produced stress concentration that increases the residual stress by 64.7%, which caused the hardness to rebound. Additionally, surface composition and microstructure evolution led to the first decrease and then increase of friction coefficient. Due to grain refinement and residual stress induced by force effect, the hardness improved continuously and the friction coefficient decreased continuously. Understanding the influence mechanism of laser action modes on surface structure/property can furnish theoretical guidance for development of laser processing technology in the industrial field.